The rules are simple. I start out with one sentence of a story and the next user continues it and continues it. You may have to read back a few pages if you're writing in like the fifth page or whatever to get the jist of the story. Try not to make the story too random or separate for each user. You can add your own twists and stuff, but try to make it all relevant and coherent. The story will end when there is a conclusion of some sort.

Starting off:

It was a dark and stormy night.

-EDIT
You can add paragraphs instead to make it go faster.